A. Multiple antibiotic resistance in a number of gram-negative bacteria is mediated by the expression of a class of transcriptional activators related to the archetypal AraC protein. In Escherichia coli, inactivation of the MarR repressor results in expression of one such activator, MarA, which in turn activates a number of genes that generate resistance to antibiotics, superoxides and organic solvents. In spite of the importance of AraC-like proteins in gene regulation, no high resolution structure has yet been determined which would shed light on how the activators bind DNA and how they activate transcription. In collaboration with R. G. Martin, S. Rhee and D. Davies, progress toward solving the structure of MarA bound to a mar promoter DNA oligonucleotide has been made: co-crystals which defract to 2.9 A have been obtained as have isomorphous crystals substituted with iodine and selenium. Functional studies of MarA transcriptional activation are also being carried out. We have found that MarA not only activates the mar promoter by binding to it, but also enables Fis, a small DNA binding protein, to act as an accessory activator for the promoter. In addition, we have obtained mutants of MarA that stimulate mar transcription but cannot be further stimulated by Fis. We have postulated that these mutants of MarA might bind to the promoter but not bend it appropriately. We are currently testing transcriptional activation by these mutant MarA proteins in vitro. Other studies are focusing on different modes of action of MarA when binding close to, or distant from, the -35 hexamers of particular promoters. B. Our investigation of the INH (isoniazid) susceptibility of certain peroxidase-deficient mutants of E. coli, shows that there are two lines of defense to peroxidative damage in the bacteria. The first is the OxyR-regulated peroxidases which dissipate endogenously formed peroxides. The second is the RecA system which repairs DNA damage by peroxides that was unaffected by the peroxidases. Bacteria lacking both systems are non-viable but our discovery of a suppressor that renders such cells viable suggests the existence of a cryptic system for preventing peroxidative damage.